Method of producing a lithium ion secondary battery and a lithium ion secondary battery produced thereby

ABSTRACT

A paste-like active material mixture prepared by mixing an active material powder and a particulate material comprising a polymer soluble in a nonaqueous electrolytic solution is applied to, e.g., collectors  1   c  and  2   c  to a uniform thickness, and then dried to form positive and negative electrodes  1, 2  containing an active material powder and a particulate polymer. The two electrodes are assembled into an electrode laminate into which the foregoing electrolytic solution is then injected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing a lithium ionsecondary battery comprising a nonaqueous electrolytic solution and moreparticularly to a method of producing and the structure of a safelithium ion battery having a high charge-discharge efficiency comprisinga low fluidity or gelled electrolytic solution.

2. Description of the Related Art

Portable electronic apparatus have found a very great demand for reducedsize and weight. The accomplishment of the demand greatly depends on theenhancement of the properties of the battery to be mounted in theseportable electronic apparatus. In order to meet this demand, thedevelopment and improvement of various batteries are under way. Inparticular, a lithium battery is a secondary battery which can realizethe highest voltage, energy density and load resistance in the existingbatteries. The improvement of lithium batteries is still under way.

FIG. 1 is a schematic sectional view illustrating the structure of anordinary lithium ion secondary battery which has been put into practicaluse. The lithium ion secondary battery comprises as essentialconstituents a positive electrode 1, a negative electrode 2, and anionically-conducting layer 3 provided interposed therebetween. In thislithium ion secondary battery, as the positive electrode 1 there is useda plate-like material prepared by applying a mixture of an activepositive electrode material powder 1 a such as lithium-cobalt compositeoxide, an electrically-conducting powder 1 b and a binder resin to apositive electrode collector 1 c. Similarly, as the negative electrode 2there is used a plate-like material prepared by applying a mixture of acarbon-based negative electrode active material powder 2 a and a binderresin to a negative electrode collector 2 c made of copper. As theionically-conducting layer 3 there is used a separator made of a porousfilm of polyethylene or polypropylene filled with a nonaqueouselectrolytic solution containing lithium ion. The battery structure ofthe present example comprises a single electrode laminate 4 having aseparator laminated with an electrode.

The lithium ion battery comprising such a nonaqueous electrolyticsolution is liable to rise in the danger of sparking, heat generation,etc. due to internal or external shortcircuiting caused by the rise inbattery capacity. The rise in battery capacity faces a greatapprehension that the battery might ignite. The elimination of thisdanger can be effectively accomplished by the reduction of the fluidityof the electrolytic solution. However, since the lithium ion batterycomprises a porous electrode formed by coagulating a particulate activematerial, it is very difficult for an electrolytic solution having areduced fluidity to fill thoroughly microvoids in the electrode. On theother hand, it is necessary that the microvoids in the electrode befilled with an electrolytic solution to improve the battery properties.

Further, gelled electrolytes have been of interest and under extensivestudy for practical use from the standpoint of the reduction of thethickness of batteries. However, the gelled electrolytes, too, cannot beeasily injected into the electrode. It is thus very difficult for thegelled electrolytes to fill thoroughly voids in the electrode. Batteriescomprising a gelled electrolyte are disclosed in U.S. Pat. No.5,460,904, “Nikkei Microdevice”, Nikkei BP, August 1996, page 136, etc.

As mentioned above, it is not easy for any electrolytic solution to fillthoroughly microvoids in the electrode. Thus, the production of such abattery faces a problem that it is difficult to fill thoroughly voids inthe electrode. This makes it impossible to provide a safe lithium ionsecondary battery having a high charge-discharge efficiency.

SUMMARY OF THE INVENTION

The present invention has been worked out as a result of the inventors'extensive studies of the filling of electrolytes. An object of thepresent invention is to provide a production process by which a safelithium ion battery having an excellent charge-discharge efficiencycomprising a low fluidity or gelled electrolytic solution can be easilyobtained and a lithium ion secondary battery having a structure that canenhance the charge-discharge efficiency thereof.

The first aspect of the method of producing a lithium ion secondarybattery is a method, which comprises the steps of:

preparing an active material mixture by mixing an active material powderwith a particulate polymer soluble in a nonaqueous electrolyticsolution;

forming electrodes comprising said active material powder andparticulate material by using the active material mixture as a rawmaterial;

assembling said electrodes into an electrode laminate;

and then injecting said electrolytic solution into said electrodelaminate.

The second aspect of the present invention is a method according to thefirst aspect of the present invention, wherein said particulate polymersoluble in a nonaqueous electrolytic solution comprises at least one ofmethacrylic polymer, acrylic polymer, polyethylene glycol, polypropyleneglycol, and a copolymer obtained by the copolymerization of thesepolymers with other monomers.

The third aspect of the present invention is a method according to thefirst aspect of the present invention, wherein the method furthercomprises the step of introducing the particulate polymer soluble in anonaqueous electrolytic solution externally into voids in said electrodebefore the step of assembling into the electrode laminate.

The fourth aspect of the present invention is a method according to thefirst aspect of the present invention, wherein the method furthercomprises the steps of:

coating said electrode with or dipping in a solution of said polymersoluble in a nonaqueous electrolytic solution;

and then drying before the step of assembling into the electrodelaminate.

The fifth aspect of the present invention is a method according to thefourth aspect of the present invention, wherein said active materialmixture further comprises a binder resin and the method furthercomprises the step of heating said electrodes at the temperature inwhich said particulate polymer is melt and said binder resin is notmelt.

The sixth aspect of the present invention is a method according to thefifth aspect of the present invention, wherein said particulate polymercomprises at least one of polyethylene glycol and polypropylene glycoland the step of heating said electrodes is the step of heating them at80° C.

The seventh aspect of the present invention is a method according to thefirst aspect of the present invention, wherein a diameter of saidparticulate polymer is not larger than 20 μm.

The eighth aspect of the present invention is a method according to theseventh aspect of the present invention, wherein a diameter of saidparticulate polymer is not larger than 5 μL m.

The ninth aspect of the present invention is a method according to thepresent invention, which comprises the steps of:

forming electrodes comprising an active material layer;

introducing a particulate polymer soluble in a nonaqueous electrolyticsolution externally into voids in said electrode;

assembling said electrodes into the electrode laminate;

and then injecting said electrolytic solution into said batterystructure.

The tenth aspect of the present invention is a method according to theninth aspect of the present invention, wherein the step of introducingthe particulate polymer comprises externally introducing a particulatepolymer soluble in said nonaqueous electrolytic solution into voids insaid electrodes which comprise active material layers formed of anactive material powder to prepare an electrode comprising saidparticulate material in voids.

The eleventh aspect of the present invention is a method according tothe tenth aspect of the present invention, wherein the step ofintroducing the particulate polymer is performed by supplying saidelectrodes into the particulate polymer and vibrating the particulatepolymer.

The twelfth aspect of the present invention is a method according to thetenth aspect of the present invention, wherein the step of introducingthe particulate polymer is performed by coating said electrode with ordipping in a solution of said polymer soluble in a nonaqueouselectrolytic solution;

and then drying before the step of assembling into the electrodelaminate.

The thirteenth aspect of the present invention is a method according tothe tenth aspect of the present invention, wherein a diameter of saidparticulate polymer is not larger than 20 μm.

The fourteenth aspect of the present invention is a method according tothe thirteenth aspect of the present invention, wherein a diameter ofsaid particulate polymer is not larger than 5 μm.

The fifteenth aspect of the lithium ion secondary battery is a batteryof the present invention, which comprises two opposing electrodes and aseparator provided interposed therebetween, and a nonaqueouselectrolytic solution retained in voids in said electrodes and saidseparator, wherein a gelling material is incorporated in said electrodesso that the viscosity or gelation degree of said nonaqueous electrolyticsolution is higher toward said separator.

The sixteenth aspect of the lithium ion secondary battery is a batteryof the fifteenth aspect, which comprises a plurality of electrodelaminates.

The seventeenth aspect of the lithium ion secondary battery is a batteryof the sixteenth aspect, wherein said plurality of electrode laminatesare formed by alternately arranging a positive electrode and a negativeelectrode between a plurality of separated separators.

The eighteenth aspect of the lithium ion secondary battery is a batteryof the sixteenth aspect, wherein said plurality of electrode laminatesare formed by alternately arranging a positive electrode and a negativeelectrode between the gap of a wound separator.

The nineteenth aspect of the lithium ion secondary battery is a batteryof the sixteenth aspect, wherein said plurality of electrode laminatesare formed by alternately arranging a positive electrode and a negativeelectrode between the gap of a folded separator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic sectional view illustrating the structure andelectrode laminate of an ordinary lithium ion battery according to thepresent invention.

FIG. 2 is a schematic sectional view illustrating the multi-layerstructure of another embodiment of the lithium ion battery according tothe present invention.

FIG. 3 is a schematic sectional view illustrating the multi-layerstructure of a further embodiment of the lithium ion battery accordingto the present invention.

FIG. 4 is a schematic sectional view illustrating the multi-layerstructure of a still further embodiment of the lithium ion batteryaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the first method of producing a lithium ion secondary battery of thepresent invention, a paste-like active material mixture prepared bymixing an active material powder and a particulate material comprising apolymer soluble in a nonaqueous electrolytic solution is applied to acollector to a uniform thickness, and then dried to prepare an electrodecomprising the active material powder and particulate material. Theelectrode thus prepared is then assembled into a battery structurecomprising an electrode laminate 4 having a separate provided interposedbetween a positive electrode and a negative electrode as shown in FIG.1. The foregoing electrolytic solution is then injected into the batterystructure.

In the case of the first method, the electrode has previously compriseda particulate polymer soluble in a nonaqueous electrolytic solutionincorporated therein, making it easy for an electrolytic solution havinga reduced viscosity to be injected into and fill voids in the electrode.The dissolution of the polymer in the electrolytic solution which hasthus been injected makes it possible to render the electrolytic solutionhighly viscous or gel the electrolytic solution. Thus, it can be easilymade possible to fill up microvoids in the electrode with a highlyviscous or gelled electrolytic solution which exhibits a reducedfluidity that can reduces danger. Further, the polymer also serves as anadhesive that can enhance the strength of the electrode. Moreover, sincethe particulate polymer is introduced into the electrode together withan active material during the preparation of the electrode, it can beincorporated in the electrode in a desired amount over the entire regionin the electrode, making it easy to adjust the viscosity and gelationdegree of the electrolytic solution.

Thus, a safe lithium ion battery having an excellent charge-dischargeefficiency comprising a low fluidity or gelled electrolytic solution canbe easily obtained.

In this case, the active material mixture preferably comprises a binderresin, an organic solvent, a electrically-conducting particulatematerial, etc. incorporated therein in a proper amount besides theactive material powder and the particulate material comprising a polymersoluble in an electrolytic solution. The present method has beendescribed with reference to the application of the active materialmixture to a collector which is then formed into an electrode plate.However, it is not always necessary to use a collector.

The second method of producing a lithium ion secondary battery of thepresent invention comprises subjecting the electrode comprising aparticulate polymer uniformly incorporated therein obtained in theforegoing first method to oscillation in a particulate materialcomprising a polymer soluble in a nonaqueous electrolytic solution orlike treatment so that the particulate polymer is externally introducedinto voids in the electrode, assembling the electrode comprising aparticulate polymer incorporated also in its voids into a batterystructure, and then injecting the nonaqueous electrolytic solution intothe battery structure.

In addition to the same effect as exerted by the foregoing first method,the third method exerts the following effects. In some detail, thefurther external introduction of a particulate polymer into theelectrode comprising a particulate polymer incorporated thereinuniformly over the entire region thereof causes the distribution of theparticulate polymer in the electrode to be scattered, making it possibleto change the viscosity or gelation degree of the electrolytic solutiondepending on the site in the electrode. By changing the viscosity orgelation degree of the electrolytic solution in voids in the electrodedepending on the site in the electrode, the charge-dischargecharacteristics of the electrode which acts as a battery can beaffected.

In other words, important factors determining the charge-dischargeefficiency of batteries include the efficiency of doping and dedoping oflithium ion during charging and discharging of active material. In anordinary battery structure, the ease of migration of lithium ion isuniform in the electrolytic solution. Therefore, doping and dedoping oflithium ion occur particularly in the vicinity of the surface of theelectrode close to the separator. Thus, the active material in theelectrode cannot be effectively used, making it impossible to providedesired charge-discharge characteristics. It is thought, on the otherhand, that as in the lithium ion secondary battery according to thepresent invention, by raising the viscosity or gelation degree of thenonaqueous electrolytic solution in the electrode toward the separator,the difference in the doping and dedoping rates in the positive andnegative electrode active material layers between on the separator sideand inside the active material can be relaxed, making it possible toeffectively use the active material inside the electrode and henceimprove the charge-discharge characteristics of the battery.

Accordingly, by raising the viscosity or gelation degree of thenonaqueous electrolytic solution in the electrode toward the separator,the charge-discharge characteristics of the battery can be furtherimproved. In accordance with the present method, a battery having theconstitution of the present invention, i.e., a safe battery whichcomprises a nonaqueous electrolytic solution having a higher viscosityor gelation degree in the electrode toward the separator and henceexhibits better charge-discharge characteristics can be easily obtained.

The fourth method of producing a lithium ion secondary battery of thepresent invention comprises coating the electrode comprising aparticulate polymer uniformly incorporated therein obtained in theforegoing first method with or dipping the electrode in a solution of apolymer soluble in a nonaqueous electrolytic solution, drying theelectrode, assembling the electrode thus dried into a battery structure,and then injecting the foregoing electrolytic solution into the batterystructure.

In the present method, the application of the polymer solution to theelectrode or the dipping of the electrode in the polymer solution makesit possible to further introduce the polymer into voids in the electrodecomprising a particulate polymer uniformly incorporated therein. Thus,the present method exerts the same effect as in the foregoing thirdmethod.

The tenth method of producing a lithium ion secondary battery of thepresent invention comprises forming an active material powder into anelectrode, subjecting the electrode to oscillation in a particulatematerial comprising a polymer soluble in a nonaqueous electrolyticsolution or like treatment so that the particulate polymer is externallyintroduced into voids in the electrode, assembling the electrodecomprising a particulate polymer incorporated in its voids into abattery structure, and then injecting the nonaqueous electrolyticsolution into the battery structure. In accordance with the presentmethod, the electrolytic solution can be rendered highly viscous orgelled. Further, the distribution of the particulate polymer in theelectrode can be scattered. Thus, the viscosity or gelation degree ofthe electrolytic solution can be changed depending on the site in theelectrode. Accordingly, a battery having the constitution of the presentinvention, i.e., a safe lithium ion battery comprising a low fluidity orgelled electrolytic solution and having an excellent charge-dischargeefficiency can be easily obtained.

The electrode formed of an active material mixture powder may comprise abinder resin, an organic solvent, an electrically-conducting particulatematerial, etc. incorporated in the active material powder as necessary.These additives may be applied to the collector which is then formedinto an electrode plate.

The twelfth method of producing a lithium ion secondary battery of thepresent invention comprises coating an electrode formed of an activematerial powder with or dipping the electrode in a solution of a polymersoluble in a nonaqueous electrolytic solution, drying the electrode,assembling the electrode thus dried into a battery structure, and theninjecting the electrolyte into the battery structure. The application ofthe polymer solution to the electrode or the dipping of the electrode inthe polymer solution makes it possible to externally introduce thepolymer into voids in the electrode. Thus, the present method exerts thesame effect as in the tenth method.

As the polymer soluble in the electrolytic solution employable in thepresent invention there may be used a polyether polymer such asmethacrylic polymer, acrylic polymer, polyethylene glycol andpolypropylene glycol, a polymer obtained by the copolymerization ofthese polymers with other monomers or such a polymer comprising variousadditives such as crosslinking agent optionally incorporated therein.

The particulate polymer to be mixed with an active material powder whichis then formed into an electrode has a grain diameter of not more than20 μm, preferably not more than 5 μm. If the grain diameter of theparticulate polymer is too great, the particulate polymer is unevenlydissolved in the electrolytic solution with which the electrode isimpregnated. The grain diameter of the particulate polymer to beexternally introduced into voids in the electrode formed of an activematerial powder is not more than 1 μm, preferably not more than 0.2 μm.

As the positive electrode active material employable herein there may beused an oxide of lithium with a transition metal such as cobalt, nickeland manganese, a chalcogen compound containing lithium, a compositecompound thereof, a chalcogen compound containing the foregoing oxideand lithium, or such a composite compound comprising various additiveelements incorporated therein. As the negative electrode active materialemployable herein there may be preferably used a carbon-based compoundsuch as easily graphitized carbon, difficultly graphitized carbon,polyacene and polyacetylene or an aromatic hydrocarbon compound havingan acene structure such as pyrene and perylene. In practice, however,any materials capable of occluding and releasing lithium ion, whichplays an essential role in the battery operation, may be used. Such anactive material is used in a particulate form. The grain diameter of theactive material is preferably within a range from 0.3 to 20 μm,particularly from 1 to 5 μm. If the grain diameter of the activematerial is too small, there occur too few voids in the electrode thusformed. Further, the surface area of the active material coated by anadhesive (binder resin) during the formation of an electrode is toogreat. Thus, doping and dedoping of lithium ion duringcharging/discharging cannot be efficiently effected, deteriorating thebattery properties. On the contrary, if the grain diameter of the activematerial is too small, the thickness of the electrode cannot be easilyreduced. Further, the packing density of the active material is reduced.

As the electrolytic solution there may be used an electrolyte saltcontaining lithium dissolved in a nonaqueous solvent used inconventional batteries. Specific examples of the nonaqueous solventemployable herein include ether solvents such as dimethoxyethane,diethoxyethane, diethyl ether and dimethyl ether, and ester solventssuch as ethylene carbonate, propylene carbonate, diethyl carbonate anddimethyl carbonate. These solvents may be used singly. Alternatively,two of these same or different kinds of solvents may be used inadmixture. As the electrolyte salt to be used for the electrolyticsolution there may be used LiPF₆, LiAsF₆, LiClO₄, LiBF₄, LiCF₃SO₃,LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃ or the like.

As the collector there may be used any metal which stays stable in thebattery. For the positive electrode, aluminum is preferably used. Forthe negative electrode, copper is preferably used. The collector may beused in any form selected from the group consisting of foil, net andexpanded metal. In practice, however, the collector is preferably in aform having a great void area such as net and expanded metal becausesuch a form helps the collector to be impregnated with the electrolyticsolution after bonded to the electrode.

Possible examples of the battery structure include one comprising asingle electrode laminate having a separator laminated with an electrodeas shown in FIG. 1, a flat laminated structure obtained by laminating aplurality of electrode laminates as shown in FIG. 2, and a multi-layerstructure such as flat wound structure comprising a plurality ofelectrode laminates formed by ellipsoidally winding an electrode and aseparator as shown in FIGS. 3 and 4. The present invention can securesafety and improve the charge-discharge efficiency. Accordingly, a safeand compact multi-layer structure battery having a high charge-dischargeefficiency and a great capacity can be obtained as well.

The present invention will be further described in the followingexamples, but the present invention should not be construed as beinglimited thereto.

Embodiment 1-4

88 wt-% of LiCoO₂ as a positive electrode active material powder, 4 wt-%of a particulate polymer set forth in Table 1 as a particulate materialcomprising a polymer soluble in a nonaqueous electrolytic solution, and8 wt-% of a graphite powder (KS-6, available from Lonza Japan Ltd.) asan electrically-conducting particulate material were mixed. The mixturewas then mixed with a polyvinylidene fluoride as a binder resin toprepare a positive electrode active material mixture. The activematerial mixture thus obtained was then applied to an aluminum foilhaving a thickness of 20 μm as a collector to a controlled thickness ofabout 100 μm by doctor blade coating method to prepare a positiveelectrode.

96 wt-% of a mesophase microbead carbon (trade name; OSAKA GAS CO.,LTD.) as a negative electrode active material powder, and 4 wt-% of aparticulate polymer set forth in Table 1 as a polymer soluble in anelectrolytic solution were mixed. The mixture was then mixed with apolyvinylidene fluoride as a binder resin to prepare a negativeelectrode active material mixture. The active material mixture was thenapplied to a copper foil having a thickness of 12 μm to a controlledthickness of about 100 μm by doctor blade coating method to prepare anegative electrode.

A separator (Cellguard #2400, available from Hoext Seraneed) wasinterposed between the two electrodes to prepare an electrode laminate.An electrolytic solution comprising lithium hexafluorophosphate as anelectrolyte dissolved in a mixture of ethylene carbonate and1,2-dimethoxyethane was then injected into the electrode laminate.During this procedure, the electrode laminate was held in place so thatthe components were not separated from each other. Extra electrolyticsolution was wiped form the electrode laminae. The electrode laminatewas packaged by an aluminum-laminated film, and then sealed to completea battery.

In the battery thus prepared, the electrolytic solution formed a stablegel without liberating from the electrode depending on temperature orwith time. The particulate polymethyl methacrylate used in Embodiments 1and 2 had a small grain diameter and a low melting point and weredissolved even at ordinary temperatures and thus gelled the electrolyticsolution. On the contrary, the particulate polyethylene glycol used inEmbodiment 3 and the particulate polyacrylonitrile used in Embodiment 4had a large grain diameter and could be difficultly dissolved atordinary temperatures. Thus, after impregnation with the electrolyticsolution, these compounds were dissolved at a temperature at which thepolymers can be dissolved but the binder resin cannot be dissolved,i.e., 80° C.

These batteries were then evaluated for properties. As shown in Table 1,all these batteries exhibited a high electrical conductivity and a highenergy density, demonstrating that the electrodes had been fairly filledwith the gelled electrolytic solution free of bubbles, etc.

TABLE 1 Particulate polymer (grain Electrical Energy density diameter)conductivity per weight Embodiment 1 Polymethyl 2 × 10⁻⁵ S/cm 120 Wh/kgmethacrylate (0.35 μm) Embodiment 2 Polymethyl 4 × 10⁻⁵ S/cm 120 Wh/kgmethacrylate (0.25 μm) Embodiment 3 Polyethylene 5 × 10⁻⁵ S/cm  90 Wh/kgglycol* (approx. 3 μm) Embodiment 4 Polyacrylo- 2 × 10⁻⁵ S/cm 100 Wh/kgnitrile* (approx. 3 μm) *Dissolved at 80° C. after impregnation withelectrolytic solution

Embodiment 5

87 wt-% of LiCoO₂, 8 wt-% of a graphite powder (KS-6, available fromLonza Japan Ltd.), and 5 wt-% of a polystyrene powder as a binder resinwere mixed. To the mixture thus obtained were then added toluene and2-propanol in a proper amount to prepare a paste-like mixture. Thepaste-like mixture was then applied to an aluminum foil having athickness of 20 μm as a collector to a controlled thickness of about 100μm by doctor blade coating method to prepare a positive electrode.

95 wt-% of mesophase microbead carbon (trade name; OSAKA GAS CO., LTD.)and 5 wt-% of a polystyrene powder were mixed. To the mixture were thenadded toluene and 2-propanol in a proper amount to prepare a paste-likemixture. The paste-like mixture was then applied to a copper foil havinga thickness of 12 μm as a collector to a controlled thickness of about100 μm by doctor blade coating method to prepare a negative electrode.

The positive electrode and negative electrode thus prepared were thensubjected to oscillation in a particulate polymethyl methacrylate havinga grain diameter of 0.25 μm (available from Soken Chemical & EngineeringCo., Ltd.) as a particulate material comprising a polymer soluble in anonaqueous electrolytic solution. In this manner, the particulatepolymer was introduced into voids in the positive electrode and negativeelectrode. The positive electrode and negative electrode comprising theparticulate polymer incorporated therein were then assembled into abattery in the same manner as in the preceding embodiments. The batterythus prepared exhibited an electrical conductivity of 2×10⁻⁵ S/cm and anenergy density per weight of 120 Wh/kg, demonstrating that theelectrodes had been fairly filled with the gelled electrolytic solutionfree of bubbles, etc.

In the present embodiment, the particulate polymethyl methacrylate asparticulate polymer occurred in the electrode little toward thecollector but much toward the other side. Thus, the distribution of theparticulate polymer in the electrode showed a density gradient that itoccurs less toward the collector side but much toward the other side.Accordingly, the electrolytic solution in the electrode showed highergelation degree and concentration toward the surface thereof oppositethe collector. In this arrangement, the battery exhibited improvedcharge-discharge characteristics as compared with batteries comprisingan electrolytic solution uniformly distributed in the electrode withrespect to gelation degree.

Embodiments 6-8

The same positive electrode and negative electrode as prepared inEmbodiment 5 were dipped in a polymer solution soluble in a nonaqueouselectrolytic solution set forth in Table 2, withdrawn, and then dried.If the polymer solution had a high viscosity, extra solution was wipedfrom the electrodes which had been withdrawn. In this manner, thepolymer was introduced into voids in the positive electrode and negativeelectrode. The positive electrode and negative electrode comprising thepolymer incorporated therein were then assembled into a battery in thesame manner as in the preceding embodiment. As shown in Table 2, allthese batteries exhibited a high electrical conductivity and a highenergy density, demonstrating that the electrodes had been fairly filledwith the gelled electrolytic solution free of bubbles, etc. Theparticulate polyethyl methacrylate used in Embodiment 7 had a greatconcentration and thus could be difficultly dissolved at ordinarytemperatures. Thus, after impregnation with the electrolytic solution,the compound was dissolved at a temperature of 80° C.

The polymer distribution in the electrode could be graded as inEmbodiment 5, making it possible to raise the gelation degree andconcentration of the electrolytic solution in the electrode toward thesurface of the electrode opposite the collector.

TABLE 2 Polymer Electrical Energy density solution conductivity perweight Embodiment 6 Polymethyl 1 × 10⁻⁵ S/cm 100 Wh/kg methacrylate (5wt-% toluene solution) Embodiment 7 Polymethyl 4 × 10⁻⁶ S/cm  90 Wh/kgmethacrylate (15 wt-% toluene solution)* Embodiment 8 Polyethylene 5 ×10⁻⁵ S/cm 100 Wh/kg glycol (5 wt-% aqueous solution) *Dissolved at 80°C. after impregnation with electrolytic solution

By externally introducing a polymer into an electrode formed byuniformly mixing the active material powder and 5 particulate polymer asused in Embodiments 1 to 4 in the same manner as in Embodiments 5 to 8,the polymer can be incorporated in the electrode also on the collectorside and the polymer distribution in the electrode can be graded as inEmbodiments 5 to 8.

Embodiment 9

A piece having a predetermined size was stamped out of the same negativeelectrode and positive electrode as prepared in Embodiment 5 and aseparator (Cellguard #2400, available from Hoext Seraneeds). Thesepieces were then repeatedly superimposed on each other to form aplurality of separator-negative electrode-separator-positive electrodelaminates. Thus, a flat laminated battery as shown in FIG. 2 wasprepared. Collector tabs connected to the edge of the positive electrodeand the negative electrode in the flat laminated battery were thenspot-welded to each other, respectively, to connect the foregoing flatlaminated batteries in parallel to each other. An electrolytic solutioncomprising lithium hexafluorophosphate as an electrolyte dissolved in amixture of ethylene carbonate and 1,2-dimethoxyethane was then injectedinto the battery. Extra electrolytic solution was then wiped from thebattery. The battery was packaged by an aluminum-laminated film. Thebattery was then sealed under reduced pressure in such a manner that noair layers were interposed between the electrodes to obtain amulti-layer battery.

As a result, a stable gel was formed without causing the electrolyticsolution to be liberated from the electrodes depending on temperature orwith time as in the single-layer battery of Embodiment 5. The batteryexhibited a high electrical conductivity and a high energy density perweight, demonstrating that the electrodes had been fairly filled withthe gelled electrolytic solution free of bubbles, etc. Further, themulti-layer structure provided a compact lithium ion secondary batteryhaving a raised capacity.

Embodiment 10

The same belt-like negative electrode as prepared in Embodiment 5 wasinterposed between two sheets of separators (Cellguard #2400, availablefrom Hoext Seraneeds). One end of the separators having the negativeelectrode interposed therebetween was then folded by a predeterminedamount. A positive electrode having a predetermined size prepared in thesame manner as in Embodiment 1 was then inserted into the gap of thefolded part. The laminate was then passed through a laminator.Subsequently, another sheet of the positive electrode having apredetermined size was placed on the position opposed to the foregoingpositive electrode which had been inserted into the gap of the foldedpart. The foregoing belt-like separators were then ellipsoidally woundhalf round in such an arrangement that the latter positive electrode wasinterposed therebetween. The separator was further wound with a furthersheet of the positive electrode being interposed therebetween. Thisprocedure was then repeated to prepare a flat wound laminated batteryhaving a plurality of electrode laminates as shown in FIG. 3. Collectortabs connected to the edge of the sheets of the positive electrode inthe flat wound laminated battery were spot-welded to each other toelectrically connect the plurality of electrode laminates in parallel toeach other. An electrolytic solution comprising lithiumhexafluorophosphate as an electrolyte dissolved in a mixture of ethylenecarbonate and 1,2-dimethoxyethane was then injected into the battery.The battery was then packaged by an aluminum-laminated film. The batterywas then sealed under reduced pressure in such a manner that no airlayers were interposed between the electrodes to obtain a multi-layerbattery. As a result, a compact lithium ion secondary battery having ahigh energy density, excellent charge-discharge properties, a highcapacity and a high safety was obtained as in Embodiment 9.

The present embodiment has been described with reference to theprocedure which comprises winding belt-like separators having abelt-like negative electrode inserted therebetween with a plurality ofpositive electrodes having a predetermined size being inserted betweenthe gap thus formed. However, belt-like separators having a belt-likepositive electrode inserted therebetween may be wound with a pluralityof negative electrodes having a predetermined size being insertedbetween the gap thus formed.

Further, the present invention has been described with reference to theprocedure which comprises winding separators. However, belt-likeseparators having a belt-like negative electrode or positive electrodeinserted therebetween may be folded with a positive electrode ornegative electrode having a predetermined size being inserted betweenthe gap thus formed.

Embodiment 11

A belt-like negative electrode prepared in the same manner as inEmbodiment 5 was interposed between two sheets of separators (Cellguard#2400, available from Hoext Seraneeds). A belt-like positive electrodeprepared in the same manner as in Embodiment 1 was then placed on theouter surface one of the two sheets of separators protruding by apredetermined amount. The laminate was then passed through a laminatorwith one end of the positive electrode ahead by a predetermined amount,followed by the laminate of positive electrode, separator, negativeelectrode and separator, to form a belt-like laminate. Thereafter, theprotruding positive electrode was folded. The laminate was thenellipsoidally wound in such a manner that the positive electrode thusfolded was contained inside to prepare a flat wound laminated batteryhaving a plurality of electrode laminates as shown in FIG. 4. Anelectrolytic solution comprising lithium hexafluorophosphate as anelectrolyte dissolved in a mixture of ethylene carbonate and1,2-dimethoxyethane was then injected into the battery. The battery wasthen packaged by an aluminum-laminated film. The battery was then sealedunder reduced pressure in such a manner that no air layers wereinterposed between the electrodes to obtain a multi-layer battery. As aresult, a compact lithium ion secondary battery having a high energydensity, excellent charge-discharge properties, a high capacity and ahigh safety was obtained as in Embodiments 9 and 10.

The present embodiment has been described with reference to theprocedure which comprises winding belt-like separators having abelt-like negative electrode interposed therebetween with a positiveelectrode placed on the outer side of one of the separators. However,belt-like separators having a belt-like positive electrode interposedtherebetween may be wound with a negative electrode placed on the outerside of one of the separators.

In Embodiments 9 to 11, the number of laminates was varied. As a result,the battery capacity increased in proportion to the number of laminates.The use of the same electrodes as prepared in Embodiments 6 to 8 made itpossible to obtain safe batteries having excellent charge-dischargeproperties and a high capacity as in Embodiments 9 to 11. Further, theuse of electrodes having a gradient in the polymer distribution thereinas in Embodiments 1 to 4 made it possible to further improve thecharge-discharge properties of the battery.

In accordance with the first and second processes for the production ofa lithium ion secondary battery of the present invention, whichcomprises using an active material mixture prepared by mixing an activematerial powder with a particulate material comprising a polymer solublein a nonaqueous electrolytic solution to prepare an electrode comprisingsaid active material powder and particulate material, assembling saidelectrode into a battery structure, and then injecting said electrolyticsolution into said battery structure, a safe lithium ion batterycomprising a low fluidity or gelled electrolytic solution and having anexcellent charge-discharge efficiency can be easily obtained.

In accordance with the third method of producing a lithium ion secondarybattery of the present invention, which comprises externally introducinga particulate material comprising a polymer soluble in a nonaqueouselectrolytic solution into voids in the electrode before the assemblyinto a battery structure in the first production process, the followingeffects can be added to the foregoing effects. In some detail, theviscosity or gelation degree of the electrolytic solution can be changeddepending on the site in the electrode. In this arrangement, theviscosity or gelation degree of the nonaqueous electrolytic solution inthe electrode can be raised toward the separator, making it possible tofurther improve the charge-discharge properties of the battery.

The fourth to ninth method of producing a lithium ion secondary batteryof the present invention comprises applying a solution of a polymersoluble in a nonaqueous electrolytic solution to the electrode ordipping the electrode in the polymer solution, and then drying theelectrode before the assembly into a battery structure in the firstproduction process to exert the same effects as in the second productionprocess.

In accordance with the tenth and eleventh processes for the productionof a lithium ion secondary battery of the present invention, whichcomprises externally introducing a particulate material comprising apolymer soluble in a nonaqueous electrolytic solution into voids in anelectrode formed of an active material powder to prepare an electrodecomprising said particulate material in voids, assembling said electrodeinto a battery structure, and then injecting said electrolytic solutioninto said battery structure, the viscosity or gelation degree of theelectrolytic solution can be raised. Further, the viscosity or gelationdegree of the electrolytic solution in the electrode can be raisedtoward the separator, making it easy to obtain a safe lithium ionbattery having an excellent charge-discharge efficiency.

The twelfth method of producing a lithium ion secondary battery of thepresent invention comprises coating an electrode formed of an activematerial powder with or dipping said electrode in a solution of apolymer soluble in a nonaqueous electrolytic solution, drying saidelectrode, assembling said electrode into a battery structure, and theninjecting said electrolytic solution into said battery structure toexert the same effects as in the tenth production process.

The first structure of the lithium ion secondary battery of the presentinvention comprises an electrode laminate comprising two opposingelectrodes and a separator provided interposed therebetween, and anonaqueous electrolytic solution retained in voids in the electrodes andthe separator, characterized in that a gelling material is incorporatedin the electrodes so that the viscosity or gelation degree of thenonaqueous electrolytic solution is higher toward the separator. In thisarrangement, the difference in the doping and dedoping rates in thepositive and negative electrode active material layers between on theseparator side and inside the active material can be relaxed, making itpossible to effectively use the active material inside the electrode andhence improve the charge-discharge efficiency of the battery.

The second to fifth structures of the lithium ion secondary battery ofthe present invention comprises a plurality of electrode laminates inaddition to the first structure. This structure can provide a compactmulti-layer structure lithium ion secondary battery having a highcharge-discharge efficiency and a great capacity.

What is claimed is:
 1. A method of producing a lithium ion secondarybattery, which comprises the steps of: preparing a positive electrodeactive material mixture by mixing a positive electrode active materialpowder with a particulate polymer soluble in a nonaqueous electrolyticsolution; preparing a negative electrode active material mixture bymixing a negative electrode active material powder with a particulatepolymer soluble in a nonaqueous electrolytic solution: forming apositive electrode comprising said positive electrode active materialpowder and said particulate polymer soluble in a nonaqueous electrolyticsolution by coating the positive electrode active material mixture ontoa metal substrate; forming a negative electrode comprising said negativeelectrode active material powder and particulate polymer soluble in anonaqueous electrolytic solution by coating the negative electrodeactive material mixture onto a metal substrate; assembling said positiveelectrode and said negative electrode into an electrode laminate; andthen injecting said electrolytic solution into said electrode laminate.2. The method of producing a lithium ion secondary battery according toclaim 1, wherein said particulate polymer soluble in a nonaqueouselectrolytic solution comprises at least one of methacrylic polymer,acrylic polymer, polyethylene glycol, polypropylene glycol, and acopolymer obtained by the copolymerization of these polymers with othermonomers.
 3. The method of producing a lithium ion secondary batteryaccording to claim 1, further comprising the step of introducing theparticulate polymer soluble in a nonaqueous electrolytic solutionexternally into voids in said positive electrode and said negativeelectrode before the step of assembling said positive electrode and saidnegative electrode into the electrode laminate.
 4. The method ofproducing a lithium ion secondary battery according to claim 1, furthercomprising the steps of: coating said positive electrode and saidnegative electrode with or dipping said positive electrode and saidnegative electrode in a solution of said polymer soluble in a nonaqueouselectrolytic solution; and then drying said coated positive electrodeand said negative electrode before the step of assembling said positiveelectrode and said negative electrode into the electrode laminate. 5.The method of producing a lithium ion secondary battery according toclaim 4, wherein said active material mixture further comprises a binderresin and the method further comprises the step of heating said positiveelectrode and said negative electrode at a temperature at which saidparticulate polymer soluble in a nonaqueous electrolytic solution meltsand said binder resin does not melt.
 6. The method of producing alithium ion secondary battery according to claim 5, wherein saidparticulate polymer comprises at least one polymer selected from thegroup consisting of polyethylene glycol and polypropylene glycol andsaid electrodes are heated at 80° C.
 7. The method of producing alithium ion secondary battery according to claim 1, wherein a diameterof said particulate polymer is not larger than 20 μm.
 8. The method ofproducing a lithium ion secondary battery according to claim 7, whereina diameter of said particulate polymer is not larger than 5 μm.